It is becoming popular for a hybrid vehicle which uses both engine and motor/generator as a running drive source or for an electric vehicle which uses the motor/generator as a driving drive source to accumulate electric energy regenerated from the kinetic energy by the motor/generator during braking operation thereby to improve the fuel efficiency. In this sense, the motor/generator is considered to be the regeneration brake device which applies regeneration braking force to the driving vehicle wheels. Since sufficient braking force cannot be obtained by the regeneration brake device alone, normally a device for generating a basic hydraulic pressure braking force by a conventional hydraulic brake device, a hydraulic brake device which is driven by the operation fluid supplied from the master cylinder device is provided at each vehicle wheel to supplement the insufficient braking force.
According to thus structured vehicle brake system, the regeneration braking force obtained by the regeneration brake device changes the amount depending on some factors such as the battery charged state. Accordingly, the hydraulic braking force to be generated by the hydraulic brake device cannot be determined only by the depression amount (operation amount) of the brake pedal by an operator and variably controlling the hydraulic braking pressure is required. For this type of device suitable for variable control, an example of such device is disclosed in a Patent Document 1, in which an input piston and an output piston are provided and the relative distance therebetween can be variably changed.
According to the master cylinder device in the Patent Document 1, a housing, a pressurizing piston (output piston), an input piston and a reaction force applying mechanism are provided. In addition thereto, an operation force depending pressure realizing mechanism is also equipped. Normally the master cylinder device is operated under the driving hydraulic pressure operating state where the master cylinder pressure is generated by pressurizing the operation fluid in the pressure chamber by the pressurizing piston depending on the hydraulic pressure of a high pressure source (driving hydraulic pressure generating portion). On the other hand in a state that the pressure in the high pressure source is insufficient, the operation force depending pressure realizing mechanism stops operation of the reaction force applying mechanism and at the same time allows the transmission of operation force from the input piston to the pressurizing piston. Then the pressurizing piston is operated depending on the operation force under the operation force operating state where the master master cylinder pressure is generated.